varargs

Synopsis

Description

This set of macros allows portable procedures that accept variable argument lists
to be written. Routines that have variable argument lists (such as printf(3C)) but
do not use varargs are inherently non-portable, as different machines use different
argument-passing conventions.

va_alist is used as the parameter list in a function header.

va_dcl is a declaration for va_alist. No semicolon should follow va_dcl.

va_list is a type defined for the variable used to traverse the
list.

va_start is called to initialize pvar to the beginning of the list.

va_arg will return the next argument in the list pointed to by
pvar. type is the type the argument is expected to be. Different
types can be mixed, but it is up to the routine to
know what type of argument is expected, as it cannot be determined at
runtime.

va_end is used to clean up.

Multiple traversals, each bracketed by va_start and va_end, are possible.

See Also

Notes

It is up to the calling routine to specify in some manner
how many arguments there are, since it is not always possible to
determine the number of arguments from the stack frame. For example, execl
is passed a zero pointer to signal the end of the list.
printf can tell how many arguments are there by the format.

It is non-portable to specify a second argument of char, short, or
float to va_arg, since arguments seen by the called function are not
char, short, or float. C converts char and short arguments to int and
converts float arguments to double before passing them to a function.